Growing energy needs have prompted specialists to take cognizance of the fact that the traditional energy resources, such as coal, petroleum or natural gas, are not inexhaustible, or at least that they are becoming costlier all the time, and that it is advisable to consider replacing them gradually with other energy sources, such as nuclear energy, solar energy, or geothermal energy. Hydrogen, too, is coming into use as an energy source.
Hydrogen may be used, for example, as fuel for internal-combustion engines in place of hydrocarbons. In this case it has the advantage of eliminating atmospheric pollution through the formation of oxides of carbon or of sulfur upon combustion of the hydrocarbons. Hydrogen may also be used to fuel hydrogen-air fuel cells for production of the electricity needed for electric motors.
One of the problems posed by the use of hydrogen is its storage and transportation. A number of solutions have been proposed. Hydrogen may be stored under high pressure in steel cylinders, but this approach has the drawback of requiring hazardous and heavy containers which are difficult to handle (in addition to having a low storage capacity of about 1% by weight). Hydrogen may also be stored in cryogenic containers, but this entails the disadvantages associated with the use of cryogenic liquids; such as, for example, the high cost of the containers, which also require careful handling. There are also "boil off" losses of about 2-5% per day.
Another method of storing hydrogen is to store it in the form of a hydride, which then is decomposed at the proper time to furnish hydrogen. The hydrides of iron-titanium, lanthanum-nickel, vanadium, and magnesium have been used in this manner, as described in French Pat. No. 1,529,371.
The MgH.sub.2 --Mg system is the most appropriate of all known metal-hydride and metal systems that can be used as reversible hydrogen-storage systems because it has the highest percentage by weight (7.65% by weight) of theoretical capacity for hydrogen storage and hence the highest theoretical energy density (2332 Wh/kg; Reilly & Sandrock, Spektrum der Wissenschaft, Apr. 1980, 53) per unit of storage material.
Although this property and the relatively low price of magnesium make the MgH.sub.2 --Mg seem the optimum hydrogen storage system for transportation, for hydrogen-powered vehicles that is, its unsatisfactory kinetics have prevented it from being used up to the present time. It is known for instance that pure magnesium can be hydrided only under drastic conditions, and then only very slowly and incompletely. The dehydriding rate of the resulting hydride is also unacceptable for a hydrogen storage material (Genossar & Rudman, Z. f. Phys. Chem., Neue Folge 116, 215 [1979], and the literature cited therein).
Moreover, the hydrogen storage capacity of a magnesium reserve diminishes during the decomposition-reconstitution cycles. This phenomenon may be explained by a progressive poisoning of the surface, which during the reconstitution renders the magnesium atoms located in the interior of the reserve inaccessible to the hydrogen.
To expel the hydrogen in conventional magnesium or magnesium/nickel reserve systems, temperatures of more than 250.degree. C. are required, with a large supply of energy at the same time. The high temperature level and the high energy requirement for expelling the hydrogen have the effect that, for example, a motor vehicle with an internal combustion engine, cannot exclusively be operated from these stores. This occurs because the energy contained in the exhaust gas, in the most favorable case (full load), is sufficient for meeting 50% of the hydrogen requirement of the internal combustion engine from a magnesium or magnesium/nickel store. Thus, the remaining hydrogen demand must be taken from a hydride store. For example, this store can be titanium/iron hydride (a typical low-temperature hydride store) which can be operated at temperatures down to below 0.degree. C. These low-temperature hydride stores have the disadvantage of only having a low hydrogen storage capacity.
Storage materials have been developed in the past, which have a relatively high storage capacity but from which hydrogen is nevertheless expelled at temperatures of up to about 250.degree. C. U.S. Pat. No. 4,160,014 describes a hydrogen storage material of the formula Ti.sub.[1-x] Zr.sub.[x] Mn.sub.[2-y-z] Cr.sub.[y] V.sub.[z], wherein x=0.05 to 0.4, y=0 to 1 and z=0 to 0.4. Up to about 2% by weight of hydrogen can be stored in such an alloy. In addition to this relatively low storage capacity, these alloys also have the disadvantage that the price of the alloy is very high when metallic vanadium is used.
Moreover, U.S. Pat. No. 4,111,689 has disclosed a storage alloy which comprises 31 to 46% by weight of titanium, 5 to 33% by weight of vanadium and 36 to 53% by weight of iron and/or manganese. Although alloys of this type have a greater storage capacity for hydrogen than the alloy according to U.S. Pat. No. 4,160,014, hereby incorporated by reference, they have the disadvantage that temperatures of at least 250.degree. C. are necessary in order to completely expel the hydrogen. At temperatures of up to about 100.degree. C., about 80% of the hydrogen content can be discharged in the best case. However, a high discharge capacity, particularly at low temperatures, is frequently necessary in industry because the heat required for liberating the hydrogen from the hydride stores is often available only at a low temperature level.
In contrast to other metals or metal alloys, especially such metal alloys which contain titanium or lanthanum, magnesium is preferred for the storage of hydrogen not only because of its lower material costs, but above all, because of its lower specific weight as a storage material. However, the hydriding EQU Mg+H.sub.2 .fwdarw.MgH.sub.2
is, in general, more difficult to achieve with magnesium, inasmuch as the surface of the magnesium will rapidly oxidize in air so as to form stable MgO and/or Mg(OH).sub.2 surface layers. These layers inhibit the dissociation of hydrogen molecules, as well as the absorption of produced hydrogen atoms and their diffusion from the surface of the granulate particles into the magnesium storage mass.
Intensive efforts have been devoted in recent years to improve the hydriding ability of magnesium by doping or alloying it with such individual foreign metals as aluminum (Douglass, Metall. Trans. 6a, 2179 [1975]) indium (Mintz, Gavra, & Hadari, J. Inorg. Nucl. Chem. 40, 765 [1978]), or iron (Welter & Rudman, Scripta Metallurgica 16, 285 [1982]), with various foreign metals (German Offenlegungsschriften 2 846 672 and 2 846 673), or with intermetallic compounds like Mg.sub.2 Ni or Mg.sub.2 Cu (Wiswall, Top Appl. Phys. 29, 201 [1978] and Genossar & Rudman, op. cit.) and LaNi5 (Tanguy et al., Mater. Res. Bull. 11, 1441 [1976]).
Although these attempts did improve the kinetics somewhat, certain essential disadvantages have not yet been eliminated from the resulting systems. The preliminary hydriding of magnesium doped with a foreign metal or intermetallic compound still demands drastic reaction conditions, and the system kinetics will be satisfactory and the reversible hydrogen content high only after many cycles of hydriding and dehydriding. Considerable percentages of foreign metal or of expensive intermetallic compound are also necessary to improve kinetic properties. Furthermore, the storage capacity of such systems are generally far below what would theoretically be expected for MgH.sub.2.
Traditional ambient temperature metal hydrides suffer from low gravimetric hydrogen storage densities of normally less than 2 weight percent. Potential storage alloys which have gravimetric storage densities of greater than 3 weight percent tend to require high temperatures (&gt;200.degree. C.) for desorption. Magnesium-based alloys are considered to be very promising for storage alloys due to their high potential gravimetric storage densities and the low cost of Mg. However these alloys normally have poor properties such as slow kinetics, intolerance to surface poisoning and require high temperatures, typically around 300.degree. C.
Energy Conversion Devices, Inc. has investigated and developed Mg-based hydrogen storage alloys for improved storage properties, such as rapid kinetics, high cyclability and tolerance to surface poisoning. Formation of these alloys by a mechanical alloying process has been found to be successful. The mechanical alloying produces a fine multi-phase powdered alloy which can be readily activated under mild conditions, has rapid sorption kinetics and long cycle-life with tolerance to surface poisoning. However these multi-phase alloys still require temperatures of 250-350.degree. C. to desorb all of the stored hydrogen at atmospheric pressure and have strong enthalpies of formation (.DELTA.H.sub.f), typically in the range of about -60 to -75 kJ per mol of H.sub.2.
The high temperatures and heat of desorption cannot be provided by most hydrogen-use applications. Even internal combustion engines (ICE), especially when highly optimized, may not be able to provide sufficient heat to desorb the hydrogen at the required rates. Therefore another method of heating the hydrides is needed to successfully use these Mg-based alloys as practical hydrogen storage materials. As noted the .DELTA.H.sub.f are -60 to -75 kJ/mol H.sub.2. The higher and lower heats of combustion for H.sub.2 are: 286.6 and 242.3 kJ/mol H.sub.2 respectively. Three to four times the amount of heat is released from the combustion of hydrogen than is absorbed by the desorption of hydrogen from the Mg-based hydrides. Therefore it is possible to design a system which uses a portion of the stored hydrogen to provide the necessary heat of desorption. The apparent gravimetric H.sub.2 density of the system would be approximately 2/3 the actual H.sub.2 density. For instance, an alloy with 6 wt. % H.sub.2 density would have an apparent H.sub.2 density of about 4 wt. %. The instant invention provides such a system.